2O8 EXCRETION BY THE KIDNEYS AND SKIN 



can be detected in the saliva if the bladder is the source of 

 the blood. 



Micturition. When the bladder has become moderately 

 full the desire to expel its contents arises. The act of mic- 

 turition involves relaxation of the sphincter vesica and 

 contraction of the muscular walls of the bladder aided by 

 the abdominal muscles and those of the urethra. A slight 

 contraction of the abdominal muscles compresses the blad- 

 der; after a short interval the sphincter relaxes and allows 

 the stream to pass out through the urethra. When the act 

 has been begun contraction of the bladder will suffice to 

 nearly empty the organ, but complete evacuation is finally 

 brought about by a series of convulsive contractions on the 

 part of the muscles of the abdomen. 



The center controlling the reflex nervous phenomena of 

 micturition is opposite to the fourth lumbar vertebra in the 

 spinal cord. 



THE SKIN. 



Functions. The functions of the skin from a physical 

 standpoint are sufficiently apparent. It furnishes protection 

 to the underlying parts, preserves the general contour of the 

 body, affords lodgment for afferent nerve terminations, and 

 thus establishes relations between ourselves and our sur- 

 roundings. As an organ of excretion it is very important, 

 and in fact essential to life. While various materials, such 

 as urea and CO, are thus discharged from the body, their 

 amount is more or less inconsequential, and it appears that it 

 is the action of the skin as a regulator of heat "excretion" 

 which is vital. It furnishes one of the three chief routes 

 for the discharge of water from the body, and it will be 

 seen that it is largely through the output of water that the 

 output of heat is regulated. So necessary is the skin in this 

 respect that the covering with impermeable substances of as 

 much as half the body surface is followed by death. 



The skin excretions are contained in the products of the 



